1. Field of the Invention
The present invention relates generally to a method of wireless data transmission, and in particular, to a method for improving a signal-to-noise ratio (S/N) and enhancing the information fidelity of high-bit-rate wireless optical communication signals transmitted through adverse atmospheric and environmental conditions.
2. Description of the Related Art
The present invention teaches techniques to improve wireless optical communication through adverse environmental conditions, such as clouds, fog, smog, and smoke in the atmosphere, and murky water.
Wireless communication systems today rely on microwave or radio frequency pulse rates. The bandwidth with these types of transmissions is limited and higher rates are required. Light transmission offers much higher bandwidth and speed advantages, and is now being used in telecommunications using optical fibers. Currently, an optical fiber network transmits data at a rate of 40 Gigabits/sec and will soon attain 100 Gigabits/sec rate. However, for free-space line-of-sight wireless optical communication, such as satellite-to-ground, ground-to-satellite, and ground-to-ground transmission of optical data and signals one needs to overcome the deleterious effects of signal attenuation and signal-to-noise ratio (S/N) reduction caused by multiple scattering and absorption by the transmission conditions, such as clouds, fog, smoke, smog and murky water in the environment.
As an ultra short pulse (pulse width from picoseconds to femtoseconds) of light transits through a turbid medium, the temporal profile of the pulse broadens due to scattering by the suspension of microscale particles in the medium. In the case of light pulse propagation through the atmosphere, these microscale particles could be the water droplets in the cloud. Scattering arises from the variation in the local index of refraction between the air molecules in the atmosphere and water droplets in the cloud. The temporal broadening of pulses causes adjacent pulses (in the coded pulse train used for high-speed optical communication) to overlap with one another leading to clutter and information mutilation. The present invention provides methods for overcoming this type of problem and extracting useful information based on a detailed understanding of the characteristics of the broadened pulse.
It has been pointed out (U.S. Pat. No. 5,371,368 issued Dec. 6, 1994 to Alfano et. al.; Yoo and Alfano, “Time-Resolved Coherent And Incoherent Components Of Forward Light Scattering In Random Media,” Opt. Lett. 15, 320 (1990); Wang et al. “Ballistic 2-D Imaging Through Scattering Walls Using An Ultra Fast Kerr Gate”, Science 253, 769 (1991)), that the broadened pulse comprises ballistic (coherently scattered in the incident direction), snake (paraxially scattered in the incident direction) and diffusive (multiply scattered in all directions) components (or photons), as illustrated in FIG. 1. The relative magnitude of the three components depends on the properties of the turbid medium (such as, the size and distribution of microscale scattering particles, the variation in the relative index of refraction between the suspended microscale particles (e.g., water droplets) and the intervening medium (e.g., air), scattering length, ls, and absorption length, la) and the distance through which light travels in it. The key parameters that describe the propagation of light through a scattering medium are: the scattering length, ls which is the average distance between two consecutive scattering processes, the transport mean free path, lt=ls/(1−g)−1, where g=<cos θ=s·s′> is the anisotropy factor, the average being over the phase function P(s, s′), and the absorption length, la which is the average distance over which a photon is absorbed. The transport mean free path is a parameter that describes the randomization of the direction as the incident light propagates through a turbid medium. One also defines the total attenuation length, lT that is related to ls and la by the relation lT−1=ls−1+la−1. Another relevant parameter is the visibility, or visual range, Sv. Qualitatively, it is the maximum distance from which an object can be seen by the normal human eye, and is determined primarily by the object's visual contrast with respect to the background. The minimum brightness contrast that the average human eye can distinguish is about 2%. Therefore, the visibility is equal to the distance at which the apparent brightness of the object differs by 2% from the brightness of the background. This criterion leads to visibility, Sv=3.912 lT.
A ballistic component retains the coded information and is ideal for line-of-sight wireless optical communication. Snake components carry information whose fidelity depends on the temporal slice of the broadened pulse used. Diffuse components contribute to the noise and information mutilation. The early light comprising of the ballistic and snake components is of interest for application in free-space optical communication, and as such, a major concern of the present invention is to provide methods for sorting it out.
The situation is somewhat different for longer pulses, that is, those with a duration of a nanosecond or longer than with the ultra short pulse as described above. For these pulses the diffusive component of the earlier parts of a pulse overlap with the later parts of the same pulse. The overall broadening is not as much (3 to 4 orders of magnitude for picosecond and subpicosecond pulses) as that for ultra short pulses, so the signal (ballistic and snake components) and noise (diffusive component) are not as isolated in time as illustrated in FIGS. 1 and 2, but are overlapped. The present invention pertains to improving signal-to-noise ratio of optical communication signals transmitted using nanosecond and longer pulses as well.
The intensity of the ballistic component (Ib) as a function of distance z that it travels in a turbid medium is given by Equation 1:Ib≅I0 exp(−z/ls) exp(−z/la)  (1)where, I0 is the incident intensity. The ballistic pulse retains its original direction and the average distance it travels in passing through a turbid medium of thickness L isZB≅L  (2)in ballistic timeτB=nL/c  (3)where n is the index of refraction of the medium.
The relative magnitude of the diffusive component is orders of magnitude larger than that of the ballistic component for highly scattering media and its temporal profile is much broader. The diffusive components travel an average distance (ZD) of as shown in Equation 4a.ZD=nL2/2lt  (4a)in an average travel time ofτD=nL2/2ltc  (4b)in transiting through a medium of length L.
Table 1 shows transit times for ballistic and diffusive components for traveling different distances L in media with L/lt=20. In Case 2, the diffusive energy from a pulse is spread over a long time (˜100 nsec) with 10−4 of energy as compared to a ballistic 10 ps window. In Case 3, the diffusive component spreads
TABLE 1Ballistic and diffusive average timesConditionsCase 1Case 2Case 3TimeL/lt = 20L/lt = 20L/lt = 20L = 5 cmL = 12 feet = 3.56 mL = 50 mτD1.6 nsec118 nsec1.66 μsecτB160 ps12 ns166 nsecout even further (˜1.66 μs) and peak intensity giving 10−9 overall reduction in the ballistic 10 ps window. The signal can be time gated to select out the ballistic component. For a train of pulses, the broadened diffusive wing of each pulse will overlap with some of the following pulses in the train causing clutter and masking of coded information. For thick medium, the peak of scattered pulse is delayed (see FIG. 2).
There are several salient features of the transmitted light pulse that can be used to sort out the early light and improve the signal-to-noise ratio, S/N. First, the ballistic peak and diffusive peak are shifted in time as illustrated schematically in FIGS. 1 and 2. The ballistic peak arrives at the ballistic time, τB=nL/c, while the diffusive peak arrives at τD=nL2/2ltc. The larger the value of L, the higher the separation between τB and τD will be. One can use a time gate (U.S. Pat. No. 5,140,463 issued Aug. 18, 1992 to Alfano et. al.; U.S. Pat. No. 5,371,368 issued Dec. 6, 1994 to Alfano et. al.; Gayen and Alfano, “Emerging Optical Biomedical Imaging Techniques,” Opt. Photon. News 7(3), 22 (1996)) that opens for a short interval to let the early light through, and close in time to effectively block the diffusive light.
Second, the diffusive components travel longer distances within the scattering medium than the ballistic components, and are absorbed more. A judicious selection of the wavelength enables one to reduce the diffusive components more preferentially than the ballistic components, thus enhancing the S/N. The selection of the wavelength is critical. For transmission through clouds, the wavelength needs to be near the absorption resonances of water droplets, and wavelengths near water absorption resonances in the 800–1600 nm range are possible choices.
Third, the time zone between the ballistic peak and the onset of a diffusive component defined as the information zone in FIG. 1 provides a time window to transport data encoded in a two-dimensional (2-D) parallel array instead of a serial transmission.
Fourth, the directionality of the ballistic and snake components may be used to spatially filter out the ballistic and snake components and to reduce the diffusive components. Ballistic components propagate in the incident direction, snake components deviate slightly from but are centered around the incident direction, while the diffusive components deviate farther from the incident direction. It has been shown (U.S. Pat. No. 5,710,429 issued Jan. 20, 1998 to Alfano et. al.; Dolne et. al. “IR Fourier Space Gate And Absorption Imaging Through Random Media,” Lasers Life Sci. 6, 131 (1994)) that a Fourier space gate is effective in preferential transmission of the early light characterized by low spatial frequencies, and rejection of diffusive light with higher spatial frequencies.
The polarization property of light provides an added advantage of sorting out useful early light from the noise generated by the diffusive light and the background light consisting of natural light from the sun, the moon, the stars, and other man-made sources. The background light is generally unpolarized. Scattering events depolarize an incident beam of polarized light. Consequently, if the incident light pulse is polarized, the polarization states of the different components of the transmitted pulse will be different. The ballistic component retains its original polarization, and the snake component remains partially polarized, while the multiple scattered diffusive component becomes depolarized. It has been shown (U.S. Pat. No. 5,719,399 issued Feb. 17, 1998 to Alfano et. al.; U.S. Pat. No. 5,847,394 issued Dec. 8, 1998 to Alfano et. al.; U.S. Pat. No. 5,929,443 issued Jul. 27, 1999 to Alfano et. al.; and Demos and Alfano “Temporal Gating In Highly Scattering Media By The Degree Of Optical Polarization,” Opt. Lett. 21,161 (1996)) that a polarization gate that selects out light of preferred polarization is effective in sorting out early light and discriminating against the diffusive light.
The present invention involves use of a single or a combination of different enabling characteristics of light, such as, wavelength, polarization, pulse duration, as well as selective optimal absorption of light by the intervening turbid medium to reduce the deleterious effect of scattering and enhance the relative magnitude of the information-bearing light. Time, polarization, and space gates, together with wavelength selection for optimizing reduction of noise by absorption will be used for improvement of high-bit-rate, line-of-sight wireless optical communication.